Cell Culture Media
Cell culture media provide the nutrients necessary to maintain and grow cells in a controlled, artificial and in vitro environment. Characteristics and compositions of the cell culture media vary depending on the particular cellular requirements. Important parameters include osmolality, pH, and nutrient formulations.
Media formulations have been used to cultivate a number of cell types including animal, plant and bacterial cells. Cells cultivated in culture media catabolize available nutrients and produce useful biological substances such as monoclonal antibodies, hormones, growth factors, viruses and the like. Such products have therapeutic applications and, with the advent of recombinant DNA technology, cells can be engineered to produce large quantities of these products. Thus, the ability to cultivate cells in vitro is not only important for the study of cell physiology, but is also necessary for the production of useful substances which may not otherwise be obtained by cost-effective means.
Cell culture media formulations have been well documented in the literature and a number of media are commercially available. In early cell culture work, media formulations were based upon the chemical composition and physicochemical properties (e.g., osmolality, pH, etc.) of blood and were referred to as “physiological solutions” (Ringer, S., J. Physiol. 3:380-393 (1880); Waymouth, C., In: Cells and Tissues in Culture, Vol. 1, Academic Press, London, pp. 99-142 (1965); Waymouth, C., In Vitro 6:109-127 (1970)). However, cells in different tissues of the mammalian body are exposed to different microenvironments with respect to oxygen/carbon dioxide partial pressure and concentrations of nutrients, vitamins, and trace elements; accordingly, successful in vitro culture of different cell types will often require the use of different media formulations. Typical components of cell culture media include amino acids, organic and inorganic salts, vitamins, trace metals, sugars, lipids and nucleic acids, the types and amounts of which may vary depending upon the particular requirements of a given cell or tissue type. Often, particularly in complex media compositions, stability problems result in toxic products and/or lower effective concentrations of required nutrients, thereby limiting the functional life-span of the culture media. For instance, glutamine is a constituent of almost all media that are used in culturing of mammalian cells in vitro. Glutamine decomposes spontaneously into pyrolidone carboxylic acid and ammonia. The rate of degradation can be influenced by pH and ionic conditions but in cell culture media, formation of these breakdown products often cannot be avoided (Tritsch et al., Exp. Cell Res. 28:360-364(1962)).
Wang et al. (In Vitro 14(8):715-722 (1978)) have shown that photoproducts such as hydrogen peroxide, which are lethal to cells, are produced in Dulbecco's Modified Eagle's Medium (DMEM). Riboflavin and tryptophan or tyrosine are components necessary for formation of hydrogen peroxide during light exposure. Since most mammalian culture media contain riboflavin, tyrosine and tryptophan, toxic photoproducts are likely produced in most cell culture media.
To avoid these problems, researchers make media on an “as needed” basis, and avoid long term storage of the culture media. Commercially available media, typically in dry power form, serves as a convenient alternative to making the media from scratch, i.e., adding each nutrient individually, and also avoids some of the stability problems associated with liquid media. However, only a limited number of commercial culture media are available, except for those custom formulations supplied by the manufacturer.
Although dry powder media formulations may increase the shelf-life of some media, there are a number of problems associated with dry powdered media, especially in large scale application. Production of large media volumes requires storage facilities for the dry powder media, not to mention the specialized media kitchens necessary to mix and weigh the nutrient components. Due to the corrosive nature of dry powder media, mixing tanks must be periodically replaced.
Typically, cell culture media formulations are supplemented with a range of additives, including undefined components such as fetal bovine serum (FBS) (10-20% v/v) or extracts from animal embryos, organs or glands (0.5-10% v/v). While FBS is the most commonly applied supplement in animal cell culture media, other serum sources are also routinely used, including newborn calf, horse and human. Organs or glands that have been used to prepare extracts for the supplementation of culture media include submaxillary gland (Cohen, S., J. Biol. Chem. 237:1555-1565 (1961)), pituitary (Peehl, D. M., and Ham, R. G., In Vitro 16:516-525 (1980); U.S. Pat. No. 4,673,649), hypothalamus (Maciag, T., et al., Proc. Natl. Acad. Sci. USA 76:5674-5678 (1979); Gilchrest, B. A., et al., J. Cell. Physiol. 120:377-383 (1984)), ocular retina (Barretault, D., et al., Differentiation 18:2942 (1981)) and brain (Maciag, T., et al., Science 211:1452-1454 (1981)). These types of chemically undefined supplements serve several useful functions in cell culture media (Lambert, K. J. et al., In: Animal Cell Biotechnology, Vol. 1, Spier, R. E. et al., Eds., Academic Press New York, pp. 85-122 (1985)). For example, these supplements provide carriers or chelators for labile or water-insoluble nutrients; bind and neutralize toxic moieties; provide hormones and growth factors, protease inhibitors and essential, often unidentified or undefined low molecular weight nutrients; and protect cells from physical stress and damage. Thus, serum or organ/gland extracts are commonly used as relatively low-cost supplements to provide an optimal culture medium for the cultivation of animal cells.
Methods of Production of Culture Media
Culture media are typically produced in liquid form or in powdered form. Each of these forms has particular advantages and disadvantages.
For example, liquid culture medium has the advantage that it is provided ready-to-use (unless supplementation with nutrients or other components is necessary), and that the formulations have been optimized for particular cell types. Liquid media have the disadvantages, however, that they often do require the addition of supplements (e.g., L-glutamine, serum, extracts, cytokines, lipids, etc.) for optimal performance in cell cultivation. Furthermore, liquid medium is often difficult to sterilize economically, since many of the components are heat labile (thus obviating the use of autoclaving, for example) and bulk liquids are not particularly amenable to penetrating sterilization methods such as gamma or ultraviolet irradiation; thus, liquid culture media are most often sterilized by filtration, which can become a time-consuming and expensive process. Furthermore, production and storage of large batch sizes (e.g., 1000 liters or more) of liquid culture media are impractical, and the components of liquid culture media often have relatively short shelf lives.
To overcome some of these disadvantages, liquid culture medium can be formulated in concentrated form; these media components may then be diluted to working concentrations prior to use. This approach provides the capability of making larger and variable batch sizes than with standard culture media, and the concentrated media formulations or components thereof often have longer shelf-life (see U.S. Pat. No. 5,474,931, which is directed to culture media concentrate technology). Despite these advantages, however, concentrated liquid media still have the disadvantages of their need for the addition of supplements (e.g., FBS, L-glutamine or organ/gland extracts), and may be difficult to sterilize economically.
As an alternative to liquid media, powdered culture media are often used. Powdered media are typically produced by admixing the dried components of the culture medium via a mixing process, e.g., ball-milling, or by lyophilizing pre-made liquid culture medium. This approach has the advantages that even larger batch sizes may be produced, the powdered media typically have longer shelf lives than liquid media, and the media can be sterilized by irradiation (e.g., gamma or ultraviolet irradiation) or ethylene oxide permeation after formulation. However, powdered media have several distinct disadvantages. For example, some of the components of powdered media become insoluble or aggregate upon lyophilization such that resolubilization is difficult or impossible. Furthermore, powdered media typically comprise fine dust particles which can make them particularly difficult to reconstitute without some loss of material, and which may further make them impractical for use in many biotechnology production facilities operating under GMP/GLP, USP or ISO 9000 settings. Additionally, many of the supplements used in culture media, e.g., L-glutamine and FBS, cannot be added to the culture medium prior to lyophilization or ball-milling due to their instability or propensity to aggregate upon concentration or due to their sensitivity to shearing by processes such as ball-milling. Finally, many of these supplements, particularly serum supplements such as FBS, show a substantial loss of activity or are rendered completely inactive if attempts are made to produce powdered supplements by processes such as lyophilization.
Thus, there exists a current need for rapidly dissolving nutritionally complex stable dry powder nutritive media, media supplements, media subgroups and buffers, which can be prepared in variable bulk quantities and which are amenable to sterilization particularly by ionizing or ultraviolet irradiation.